[Lsr] John Scudder's Discuss on draft-ietf-lsr-ospf-prefix-originator-10: (with DISCUSS and COMMENT)

John Scudder via Datatracker <noreply@ietf.org> Tue, 06 April 2021 21:06 UTC

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Subject: [Lsr] John Scudder's Discuss on draft-ietf-lsr-ospf-prefix-originator-10: (with DISCUSS and COMMENT)
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John Scudder has entered the following ballot position for
draft-ietf-lsr-ospf-prefix-originator-10: Discuss

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DISCUSS:
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Although the document is largely clear and well-written (thanks for that), I
was left with one burning question: what are these sub-TLVs *for*? There’s no
specification for what the router is supposed to do with them, only how to
originate them. The only clue I get is buried down in Section 5:

   The identification of the node that is originating a specific prefix
   in the network may aid in debugging of issues related to prefix
   reachability within an OSPF network.

If their purpose is to act as debugging aids, I think you should at least say
so briefly in the abstract and introduction. If they have some purpose beyond
that, it’s missing from the doc.


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COMMENT:
----------------------------------------------------------------------

1. Section 2:

   This document defines the Prefix Source OSPF Router-ID and the Prefix
   Source Router Address Sub-TLVs for inclusion of the Router ID and a
   reachable address information for the router originating the prefix
   as a prefix attribute.

I found this sentence difficult to read. I think removing the redundant word
“information” would help a little. Beyond that, it might help to break it into
a couple sentences, as in: “This document defines the Prefix Source OSPF
Router-ID and the Prefix Source Router Address Sub-TLVs. They are used,
respectively, to include the Router ID of, and a reachable address of, the
router that originates the prefix as a prefix attribute.”

2. Section 2.1:

   For intra-area prefix advertisements, the Prefix Source OSPF Router-
   ID Sub-TLV MUST be considered invalid and ignored if the OSPF Router
   ID field is not the same as Advertising Router field in the
   containing LSA.  Similar validation cannot be reliably performed for
   inter-area and external prefix advertisements.

What does it mean for the sub-TLV to be ignored? Since you haven’t specified
any processing of the Sub-TLVs, there’s seemingly no ignoring to be done
locally — so does this mean the sub-TLV isn’t even supposed to be stored?
Flooded?

3. Section 3:

   If the originating node is advertising an OSPFv2 Router Address TLV
   [RFC3630] or an OSPFv3 Router IPv6 Address TLV [RFC5329], then the
   same address MUST be used in the Router Address field of the Prefix
   Source Router Address Sub-TLV.  When the originating node is not
   advertising such an address, implementations can determine a unique
   and reachable address (i.e., advertised with the N-flag set [RFC7684]
   or N-bit set [RFC8362]) belonging to the originating node to set in
   the Router Address field.

As I read this, if there’s no Router Address TLV, then the implementation has
to use something it advertised with the N-flag set. I infer this because you
used “i.e.” (which essentially means “in other words”). If you do mean the
parenthetical to be limiting, why not make it a MUST? If you don’t mean it to
be limiting, shouldn’t it be “e.g.” or better still, “for example”?

(Looking at RFC 7684 it doesn’t seem as though it should be limiting, because
RFC 7684 § 2.1 says the N-flag is optional even for local routes.)

4. Section 3:

   When an ABR generates inter-area prefix advertisements into its non-
   backbone areas corresponding to an inter-area prefix advertisement
   from the backbone area, the only way to determine the originating
   node information is based on the Prefix Source OSPF Router-ID and
   Prefix Source Router Address Sub-TLVs present in the inter-area
   prefix advertisement originated into the backbone area by an ABR from
   another non-backbone area.  The ABR performs its prefix calculation
   to determine the set of nodes that contribute to the best prefix
   reachability.  It MUST use the prefix originator information only
   from this set of nodes.  The ABR MUST NOT include the Prefix Source
   OSPF Router-ID or the Prefix Source Router Address Sub-TLVs when it
   is unable to determine the information of the best originating node.

What is it supposed to do if there are N contributing routes but it can only
determine the information for M < N of the contributors?

Also, should “node” be “nodes” (last word of last sentence)?

5. Section 5, nit:

   Consideration should be given to the operation impact of the increase

s/operation/operational/